PS The guy you bought him from definitely needs to be told whats up if he has any other kittens. They need to be dewormed around 6 weeks and again about 2 weeks later. Most kittens are either born with worms or are passed them through their mother's milk. People like this really get my goat cause he sold you an obviously sick kitten that probably should have been treated by now and he is now YOUR problem to deal with.
Personally I would not pay for a kitten with no vet history as this kitten has not incurred much cost upon the irresponsible seller but instead he has passed those costs along to you.
/rant.